To promote Tribes Vengeance back in 2004, Tribes 1 and 2 were released for free along with CD keys. CD key distribution eventually stopped and although the games were still freeware and playable over LAN, you couldn't play over the Internet with other players.
The Tribes official servers were shut down last year and the community has stepped in over the last year or so. You can now get setup and play Tribes 1 and 2 here and here respectively. I'm pretty sure Tribes 2 works not so sure about Tribes 1 unfortunately.